1. Field of the Invention
The present invention relates to a light-emitting device utilizing electroluminescence. In particular, the present invention relates to a structure of a pixel portion which forms a display screen.
2. Description of the Related Art
Conventionally, an organic EL (Electro Luminescence) element which emits light at a luminance of 100 to 1000 cd/m2 by application of a voltage of approximately 10 V between a pair of electrodes has been known. The organic EL element can be formed to be thin and light; therefore, the organic EL element has been expected to be applied to a display device or a lighting device. However, the need for further increase in luminance or suppression of deterioration has been pointed out for practical use of the organic EL element.
A light-emitting element has been proposed, in which luminance per unit area is substantially increased by a stacked plurality of light-emitting units and application of a current with the same current density as in a case of a single-layered light-emitting unit in an organic EL element, whereby current efficiency is increased and luminance per unit area is increased (see Patent Document 1: Japanese Published Patent Application No. 2003-45676).
According to this light-emitting element in which the light-emitting units are stacked, the same luminance as that of a regular light-emitting element including a single-layered light-emitting unit can be obtained even if current density is reduced by half. For example, when luminance of n times is desired to be obtained at desired current density, if the number of light-emitting units having the same structure between electrodes is n, it is considered that the luminance of n times can be realized without increasing the current density. At this time, a driving voltage of n times or more is required; however, the light-emitting element is highly advantageous in that it can realize the luminance of n times without sacrificing the life.